U.S. patent number 11,196,948 [Application Number 16/711,991] was granted by the patent office on 2021-12-07 for photo-detection device and imaging system.
This patent grant is currently assigned to CANON KABUSHIKI KAISHA. The grantee listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Hajime Ikeda.
United States Patent |
11,196,948 |
Ikeda |
December 7, 2021 |
Photo-detection device and imaging system
Abstract
A photo-detection device in one embodiment includes: a first
semiconductor region that accumulates a signal charge based on an
incident light; a second semiconductor region that is capable of
accumulating a signal charge, the number of signal charges that can
be accumulated in the second semiconductor region being less than
the number of signal charges that can be accumulated in the first
semiconductor region; a first gate that transfers the signal charge
from the first semiconductor region to the second semiconductor
region; and a charge multiplication unit that includes a third
semiconductor region and avalanche-multiplies the signal charge
transferred from the second semiconductor region to the third
semiconductor region.
Inventors: |
Ikeda; Hajime (Yokohama,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA |
Tokyo |
N/A |
JP |
|
|
Assignee: |
CANON KABUSHIKI KAISHA (Tokyo,
JP)
|
Family
ID: |
1000005976424 |
Appl.
No.: |
16/711,991 |
Filed: |
December 12, 2019 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20200213542 A1 |
Jul 2, 2020 |
|
Foreign Application Priority Data
|
|
|
|
|
Dec 27, 2018 [JP] |
|
|
JP2018-244016 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
31/107 (20130101); H01L 27/14627 (20130101); H01L
27/14612 (20130101); H01L 27/14609 (20130101); H04N
5/3532 (20130101); H04N 13/271 (20180501); H04N
5/357 (20130101); H04N 2013/0081 (20130101); B60Q
9/008 (20130101); B60R 11/04 (20130101) |
Current International
Class: |
H04N
5/357 (20110101); H01L 27/146 (20060101); H01L
31/107 (20060101); H04N 13/271 (20180101); H04N
5/353 (20110101); B60R 11/04 (20060101); H04N
13/00 (20180101); B60Q 9/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
US. Appl. No. 16/716,668, filed Dec. 17, 2019 by Hajime Ikeda et
al. cited by applicant .
U.S. Appl. No. 16/708,079, filed Dec. 9, 2019 by Yusuke Onuki et
al. cited by applicant .
U.S. Appl. No. 16/780,666, filed Feb. 3, 2020 by Fumihiro Inui et
al. cited by applicant.
|
Primary Examiner: Ko; Tony
Attorney, Agent or Firm: Venable LLP
Claims
What is claimed is:
1. A photo-detection device comprising: a first semiconductor
region that accumulates a signal charge based on an incident light;
a second semiconductor region that is capable of accumulating a
signal charge, the number of signal charges that can be accumulated
in the second semiconductor region being less than the number of
signal charges that can be accumulated in the first semiconductor
region; a first gate that transfers the signal charge from the
first semiconductor region to the second semiconductor region; and
a charge multiplication unit that includes a third semiconductor
region and avalanche-multiplies the signal charge transferred from
the second semiconductor region to the third semiconductor
region.
2. The photo-detection device according to claim 1, wherein the
second semiconductor region is smaller than the first semiconductor
region in a plan view.
3. The photo-detection device according to claim 1, wherein the
number of the signal charges that can be accumulated in the second
semiconductor region is less than two.
4. The photo-detection device according to claim 1, wherein a
period in which the signal charge is transferred from the first
semiconductor region to the second semiconductor region and a
period in which the signal charge is transferred from the second
semiconductor region to the third semiconductor region are shorter
than a period in which the signal charge is accumulated in the
first semiconductor region, respectively.
5. The photo-detection device according to claim 1, wherein the
first gate is arranged so as to overlap a part of the first
semiconductor region and a part of the second semiconductor region
in a plan view.
6. The photo-detection device according to claim 1, wherein the
number of the signal charges simultaneously transferred from the
first semiconductor region to the second semiconductor region
caused by a change in an electric potential applied to the first
gate is less than two.
7. The photo-detection device according to claim 6, wherein an
electric potential applied to the first gate changes repeatedly at
a predetermined cycle.
8. The photo-detection device according to claim 7, wherein a peak
value in each cycle of an electric potential applied to the first
gate increases as time elapses.
9. The photo-detection device according to claim 1, wherein in
response to a change of an electric potential applied to the first
gate, an electric potential of the second semiconductor region
changes, and the signal charge is transferred from the second
semiconductor region to the charge multiplication unit.
10. The photo-detection device according to claim 1, wherein in a
period in which an electric potential applied to the first gate
changes from a high level to a low level, in response to a change
in an electric potential of the second semiconductor region, the
signal charge is transferred from the second semiconductor region
to the charge multiplication unit, and wherein the period is longer
than a period in which an electric potential applied to the first
gate changes from a low level to a high level.
11. The photo-detection device according to claim 1 further
comprising a second gate that is arranged so as to overlap a part
of the second semiconductor region and a part of the third
semiconductor region in a plan view and transfers the signal charge
from the second semiconductor region to the charge multiplication
unit.
12. The photo-detection device according to claim 11, wherein the
number of the signal charges simultaneously transferred from the
second semiconductor region to the charge multiplication unit
caused by a change in an electric potential applied to the second
gate is less than two.
13. The photo-detection device according to claim 12, wherein an
electric potential applied to the first gate and an electric
potential applied to the second gate change in accordance with a
sinewave.
14. The photo-detection device according to claim 13, wherein an
electric potential applied to the second gate changes to be shifted
by a predetermined phase with respect to an electric potential
applied to the first gate.
15. The photo-detection device according to claim 14, the
predetermined phase is a half cycle.
16. The photo-detection device according to claim 11, wherein in a
period in which an electric potential applied to the second gate
changes from a low level to a high level, the signal charge is
transferred from the first semiconductor region to the second
semiconductor region, and wherein the period is longer than a
period in which an electric potential applied to the second gate
changes from a high level to a low level.
17. The photo-detection device according to claim 1, wherein while
the first semiconductor region accumulates the signal charge based
on the incident light, a level of a reverse bias voltage which does
not cause avalanche multiplication in the charge multiplication
unit is applied to the third semiconductor region.
18. The photo-detection device according to claim 1 further
comprising a pixel signal processing unit including a detection
unit that detects an avalanche current arising in the charge
multiplication unit and a counter unit that counts the number of
times of arising of the avalanche current detected by the detection
unit.
19. The photo-detection device according to claim 18 further
comprising a first semiconductor substrate and a second
semiconductor substrate, wherein the first semiconductor region,
the second semiconductor region, the first gate, and the charge
multiplication unit are formed on a first primary face of the first
semiconductor substrate, wherein an optical member that guides an
incident light to the first semiconductor region is formed on a
second primary face side of the first semiconductor substrate, and
wherein the second semiconductor substrate on which the pixel
signal processing unit is formed is provided on a first primary
face side of the first semiconductor substrate.
20. The photo-detection device according to claim 18 further
comprising a first semiconductor substrate and a second
semiconductor substrate, wherein the first semiconductor region,
the second semiconductor region, the first gate, and the charge
multiplication unit are formed on a first primary face of the first
semiconductor substrate, wherein an optical member that guides an
incident light to the first semiconductor region is formed on a
first primary face side of the first semiconductor substrate, and
wherein the second semiconductor substrate on which the pixel
signal processing unit is formed is provided on a second primary
face side of the first semiconductor substrate.
21. An imaging system comprising: the photo-detection device
according to claim 1; and a signal processing unit that processes a
signal output from the photo-detection device.
22. A moving body comprising: the photo-detection device according
to claim 1; a distance information acquisition unit that acquires
distance information on a distance to an object, from a parallax
image based on signals from the photo-detection device; and a
control unit that controls the moving body based on the distance
information.
Description
BACKGROUND
Field
The present disclosure relates to a photo-detection device and an
imaging system.
Description of the Related Art
Conventionally, a photo-detection device using a Single-photon
Avalanche Diode (SPAD) is known. An SPAD is an element that counts
the number of incident photons by detecting an avalanche current
that arises in response to incidence of a single photon. A reverse
bias voltage above the breakdown voltage is applied to the
avalanche diode, and a current is multiplied by avalanche
multiplication. The photo-detection device including an SPAD counts
the number of times that the current multiplied by avalanche
multiplication (hereafter, referred to as "avalanche current")
exceeds a threshold.
In an SPAD disclosed in the specification of U.S. Patent
Application Publication No. 2009/0184384, generation of signal
charges caused by photon incidence and avalanche multiplication of
the generated signal charges are performed in the same region.
Specifically, generation and avalanche multiplication of signal
charges are performed inside a depletion layer in which only the
electric field directed to a charge collection region occurs. With
such a configuration, in response to generation of signal charges,
an avalanche current is generated.
An avalanche photodiode may avalanche-multiply not only signal
charges generated by photon incidence but also charges generated
due to a different factor from photon incidence (hereafter,
referred to as "unnecessary charge"). Avalanche multiplication of
an unnecessary charge causes noise.
In the SPAD disclosed in the specification of U.S. Patent
Application Publication No. 2009/0184384, generation and avalanche
multiplication of signal charges are performed in the same region.
As a period for detecting an incident light becomes longer, a
period in which a large reverse bias voltage is applied becomes
longer. As a result, unnecessary charges are avalanche-multiplied,
and the number of detected times may increase. Therefore, in the
SPAD disclosed in the specification of U.S. Patent Application
Publication No. 2009/0184384 has a problem of noise being likely to
increase.
SUMMARY
The object of the present disclosure is to provide a
photo-detection device and an imaging system that can reduce noise
due to unnecessary charges.
According to one disclosure of the present disclosure, provided is
a photo-detection device including: a first semiconductor region
that accumulates a signal charge based on an incident light; a
second semiconductor region that is capable of accumulating a
signal charge, the number of signal charges that can be accumulated
in the second semiconductor region being less than the number of
signal charges that can be accumulated in the first semiconductor
region; a first gate that transfers the signal charge from the
first semiconductor region to the second semiconductor region; and
a charge multiplication unit that includes a third semiconductor
region and avalanche-multiplies the signal charge transferred from
the second semiconductor region to the third semiconductor
region.
Further features of the present disclosure will become apparent
from the following description of exemplary embodiments with
reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating a general configuration of a
photo-detection device in a first embodiment of the present
disclosure.
FIG. 2 is an equivalent circuit diagram of a photoelectric
conversion element in the first embodiment of the present
disclosure.
FIG. 3 is a block diagram of a pixel signal processing unit in the
first embodiment of the present disclosure.
FIG. 4 is a schematic diagram of a photo-detection device in the
first embodiment of the present disclosure.
FIG. 5 is a plan view in a first primary face of the photoelectric
conversion element in the first embodiment of the present
disclosure.
FIG. 6 is a plan view in a second primary face of the photoelectric
conversion element in the first embodiment of the present
disclosure.
FIG. 7 is a sectional view of the photoelectric conversion element
in the first embodiment of the present disclosure.
FIG. 8 is a timing chart illustrating an operation of the
photo-detection device in the first embodiment of the present
disclosure.
FIG. 9 is a timing chart illustrating an operation of a
photo-detection device in a second embodiment of the present
disclosure.
FIG. 10 is a sectional view of a photoelectric conversion element
in a third embodiment of the present disclosure.
FIG. 11 is a timing chart illustrating an operation of a
photo-detection device in the third embodiment of the present
disclosure.
FIG. 12 is a timing chart illustrating an operation of a
photo-detection device in a fourth embodiment of the present
disclosure.
FIG. 13 is a sectional view of a photoelectric conversion element
in a sixth embodiment of the present disclosure.
FIG. 14 is a block diagram illustrating a general configuration of
an imaging system according to a seventh embodiment of the present
disclosure.
FIG. 15A and FIG. 15B are diagrams illustrating configuration
examples of an imaging system and a moving body according to an
eighth embodiment of the present disclosure.
DESCRIPTION OF THE EMBODIMENTS
First Embodiment
A photo-detection device in a first embodiment of the present
disclosure will be described by using FIG. 1 to FIG. 8.
FIG. 1 is a block diagram illustrating a general configuration of
the photo-detection device in the present embodiment. A
photo-detection device 1010 has a vertical select circuit 103, a
horizontal select circuit 104, a column circuit 105, a pixel unit
106, signal lines 107, an output circuit 108, and a control circuit
109.
The pixel unit 106 has a plurality of pixels 100 arranged in a
matrix. Each of the pixels 100 includes a photoelectric conversion
element 101 and a pixel signal processing unit 102. The
photoelectric conversion element 101 photoelectrically converts an
incident light into an electrical signal. The pixel signal
processing unit 102 outputs the converted electrical signal to the
column circuit 105.
Note that, in this specification, "light" may include
electromagnetic waves having any wavelengths. That is, "light" is
not limited to visible light but may include invisible light such
as an infrared ray, an ultraviolet ray, an X ray, a gamma ray, or
the like.
The control circuit 109 generates control pulses that drive the
vertical select circuit 103, the horizontal select circuit 104, and
the column circuit 105 and supplies the control pulses to each unit
thereof. Thereby, the control circuit 109 performs control of the
drive timing or the like of each unit.
The vertical select circuit 103 supplies control signals to each of
the plurality of pixels 100 based on control signals supplied from
the control circuit 109. As illustrated in FIG. 1, the vertical
select circuit 103 supplies control signals on a row basis to
respective pixels 100 via control signal lines provided on a row
basis of the pixel unit 106. In the vertical select circuit 103, a
logic circuit such as a shift register, an address decoder, or the
like may be used.
The signal lines 107 are provided on a column basis of the pixel
unit 106 and transmit signals output from the pixels 100 on a row
selected by the vertical select circuit 103 to the column circuit
105 on the post-stage of the pixels 100 as digital signals. The
column circuit 105 performs a predetermined process on a signal of
each pixel 100 input via the signal line 107. A predetermined
process is a process such as noise removal, amplification,
conversion of an output form, or the like of an input signal, for
example. To implement these functions, the column circuit 105 may
have a parallel-to-serial converter circuit or the like.
The horizontal select circuit 104 supplies, to the column circuit
105, control pulses used for sequentially outputting signals on
which a predetermined process has been performed to the output
circuit 108 based on control pulses supplied from the control
circuit 109. The output circuit 108 includes a buffer amplifier, a
differential amplifier, or the like and outputs, to a storage unit
or a signal processing unit outside the photo-detection device
1010, a signal output from the column circuit 105.
The control circuit 109 is a circuit used for supplying control
signals that control operations and their timings of the vertical
select circuit 103, the horizontal select circuit 104, the column
circuit 105, and the output circuit 108. Note that the vertical
select circuit 103, the horizontal select circuit 104, the column
circuit 105, and the output circuit 108 may be driven by control
signals supplied from the outside of the photo-detection device
1010.
In FIG. 1, the arrangement of the pixels 100 within the pixel unit
106 may be one-dimensional, or only a single pixel 100 may be
arranged. When the pixels 100 within the pixel unit 106 are divided
into several blocks, multiple sets of vertical select circuit 103,
the horizontal select circuit 104, and the column circuit 105 may
be arranged in association with each block. Further, the horizontal
select circuit 104 and the column circuit 105 may be arranged on a
column basis.
The pixel signal processing units 102 are not necessarily required
to be provided in a one-to-one manner in all the pixels 100. For
example, a single pixel signal processing unit 102 may be shared by
a plurality of pixels 100. In such a case, the pixel signal
processing unit 102 provides the function of signal processing on
each pixel by sequentially processing signals output from
respective photoelectric conversion elements 101.
Further, the pixel signal processing unit 102 may be provided on a
different semiconductor substrate from a semiconductor substrate on
which the photoelectric conversion element 101 is provided. In such
a case, it is possible to improve sensitivity by improving the
ratio of the area that can receive light by the photoelectric
conversion element 101 (aperture ratio). The photoelectric
conversion element 101 and the pixel signal processing unit 102 are
electrically connected to the signal line 107 via a connection
wiring provided on a pixel 100 basis. Each of the signal lines 107
may include n signal lines that transmit an n-bit digital signal.
Note that, in the same manner as the pixel signal processing unit
102, the vertical select circuit 103, the horizontal select circuit
104, the column circuit 105, and the signal lines 107 may be
provided on a different semiconductor substrate from a
semiconductor substrate on which the photoelectric conversion
element 101 is provided.
FIG. 2 is an equivalent circuit diagram of the photoelectric
conversion element 101 in the present embodiment. The photoelectric
conversion element 101 has a photoelectric conversion unit 1, a
first transfer gate 2, a charge accumulation unit 3, a second
transfer gate 4, a charge multiplication unit 5, a quench circuit
6, and an output node 7.
The photoelectric conversion unit 1 includes a photodiode and
generates and accumulates charge pairs in accordance with an
incident light. The first transfer gate 2 forms a MOS transistor,
and a control signal TX1 from the vertical select circuit 103 is
applied to the first transfer gate 2. In response to the MOS
transistor having the first transfer gate 2 being turned on, one or
more signal charges in the photoelectric conversion unit 1 are
transferred to the charge accumulation unit 3. Note that at least
one of a polysilicon or a metal (copper, aluminum, or the like) may
be used for the material of the gate.
The charge accumulation unit 3 is formed of an impurity diffusion
region formed in the semiconductor substrate or an electrode and
accumulates charges transferred from the first transfer gate 2. The
second transfer gate 4 forms a MOS transistor, and a control signal
TX2 from the vertical select circuit 103 is applied to the second
transfer gate 4. In response to the MOS transistor having the
second transfer gate 4 being turned on, one or more signal charges
are transferred from the charge accumulation unit 3 to the charge
multiplication unit 5.
The charge multiplication unit 5 includes an avalanche diode, and
avalanche-multiplies supplied charges to generate an avalanche
current. A predetermined electric potentials are supplied to the
anode and the cathode of the charge multiplication unit 5,
respectively. The electric potential supplied to the cathode of the
charge multiplication unit 5 is higher than the electric potential
supplied to the anode. That is, a reverse bias voltage is applied
between the anode and the cathode of the charge multiplication unit
5. In this state, once charges are supplied to the charge
multiplication unit 5, an avalanche current occurs by avalanche
multiplication.
The quench circuit (detection unit) 6 is formed of a MOS
transistor, for example, and is connected to the cathode of the
charge multiplication unit 5 and a power source voltage line. The
output node 7 is connected to the connection node of the charge
multiplication unit 5 and the quench circuit 6 and outputs a
voltage signal based on the avalanche current. The quench circuit 6
has a function of replacing a change in an avalanche current in the
charge multiplication unit 5 with a voltage signal. Furthermore,
the quench circuit 6 functions as a load circuit at the time of
avalanche multiplication and has a function of suppressing
avalanche multiplication by suppressing the current to be supplied
to the charge multiplication unit 5. In a period in which a reverse
bias voltage of such a degree that causes electronic avalanche is
applied to the charge multiplication unit 5, in response to a
single signal charge being transferred to the charge multiplication
unit 5, a current is multiplied by avalanche multiplication. The
multiplied current causes a voltage drop in the quench circuit 6,
and the electric potential of the cathode of the charge
multiplication unit 5 decreases. As a result, electronic avalanche
is no longer formed in the charge multiplication unit 5, and
avalanche multiplication in the charge multiplication unit 5 stops.
Since an electric potential VH is then supplied to the cathode of
the charge multiplication unit 5 via the quench circuit 6, the
electric potential supplied to the cathode of the charge
multiplication unit 5 returns to the electric potential VH. That
is, the operation region of the charge multiplication unit 5 enters
a Geiger mode again. Note that the quench circuit 6 may be formed
of a resistor element.
A voltage control unit 80 can control the electric potential VH and
a voltage Q in the photoelectric conversion element 101. For
example, the voltage control unit 80 can stop an avalanche
operation of the charge multiplication unit 5 by reducing the
electric potential VH and start an avalanche operation of the
charge multiplication unit 5 by increasing the electric potential
VH. Further, the voltage control unit 80 may control the voltage Q
at the gate of the quench circuit 6. For example, the voltage
control unit 80 may detect an increase in the avalanche current in
the charge multiplication unit 5 and perform feedback control to
change the voltage Q.
FIG. 3 is a block diagram of a pixel signal processing unit in the
present embodiment. The pixel signal processing unit 102 has an
inverter circuit 203, a counter circuit (counter unit) 204, and a
selector circuit 206.
The inverter circuit 203 shapes an electric potential change at the
output node 7 and outputs a pulse signal. When the electric
potential of the cathode of the charge multiplication unit 5 is
higher than a threshold of the inverter circuit 203, the output of
the inverter circuit 203 is at a low level. On the other hand, when
the electric potential of the cathode is lower than a threshold of
the inverter circuit 203, the output of the inverter circuit 203 is
at a high level. That is, a binarized pulse signal is output from
the inverter circuit 203. A pulse signal is output from the
inverter circuit 203 in accordance with whether or not there is a
signal charge avalanche-multiplied by the charge multiplication
unit 5.
The counter circuit 204 is connected to the inverter circuit 203,
counts the number of pulses output from the inverter circuit 203,
and outputs a cumulative count value. The counter circuit 204 may
be an N-bit counter (N is a positive integer), for example. In such
a case, the counter circuit 204 can count the number of pulses up
to about the N-th power of two at the maximum. The counted number
is held in the counter circuit 204 as a detection signal. Further,
a control signal RES is supplied to the counter circuit 204 from
the vertical select circuit 103 via a drive line 207. Once the
control signal RES is supplied to the counter circuit 204, the held
counted number is reset. That is, the counter circuit 204 changes
the count value in response to receiving a pulse from the inverter
circuit 203. In such a way, the counter circuit 204 counts the
number of occurrences of an avalanche current that arises in
response to at least one signal charge being transferred to the
charge multiplication unit 5 and avalanche-multiplied.
The selector circuit 206 switches electrical
connection/disconnection between the counter circuit 204 and the
signal line 107. A control signal SEL is supplied to the selector
circuit 206 from the vertical select circuit 103 via a drive line
208. In response to the control signal SEL being supplied to the
selector circuit 206, the electrical connection/disconnection
between the counter circuit 204 and the signal line 107 is switched
in accordance with the level of the control signal SEL. The
selector circuit 206 may include, for example, a transistor, a
buffer circuit used for outputting a signal to the outside of the
pixel 100, or the like. In response to the counter circuit 204 and
the signal line 107 being electrically connected, the count value
held in the counter circuit 204 is output to the signal line
107.
Note that, instead of the selector circuit 206, a switch such as a
transistor may be provided to the node between the quench circuit 6
and the charge multiplication unit 5 or between the photoelectric
conversion element 101 and the pixel signal processing unit 102.
Also in such a case, by switching connection/disconnection of the
switch, the same function as the selector circuit 206 may be
realized. Similarly, supply of the electric potential VH to be
supplied to the quench circuit 6 may be electrically switched by
using a switch such as a transistor.
When a plurality of counter circuits 204 are arranged, a plurality
of signals may be supplied to the selector circuit 206. Thereby,
when count values held in the counter circuits 204 are output to
the signal line 107, it is possible to control the output to the
signal line 107 on a counter circuit 204 basis.
The count value that is a digital signal held in the counter
circuit 204 serves as a signal used for forming a captured image.
Specifically, in the pixel unit 106 in which the plurality of
pixels 100 are arranged in a matrix, a captured image may be
acquired by a rolling shutter operation. That is, count values in
the counter circuit may be reset sequentially on a row basis, and
the count values held in the counter circuit 204 may be output
sequentially on a row basis. Further, a captured image may be
acquired by a global electronic shutter operation. In the global
electronic shutter operation, count values in the counter circuits
204 on all the pixel rows can be reset at the same time, and
detected signals held in the counter circuits 204 can be output
sequentially on a row basis.
Note that, when the global electronic shutter operation is
performed, it is preferable to add a switching unit that switches
whether or not to perform counting in the counter circuits 204 in
order to cause the time for performing counting of pulses to be the
same as respective rows. The unit that switches whether or not to
perform counting may be a switch such as a transistor, for
example.
Further, instead of the counter circuit 204, a Time to Digital
Converter (hereafter, referred to as a TDC) and a memory unit may
be provided. In such a case, the photo-detection device 1010 can
acquire the timing when a pulse is detected.
In this modified example, the occurrence timing of a pulse signal
output from the inverter circuit 203 is converted to a digital
signal by the TDC. As a reference signal used for measuring the
timing of a pulse signal, the control signal RES is supplied to the
TDC from the vertical select circuit 103 via a drive line. The TDC
references the control signal RES as a reference of time and
acquires a digital signal corresponding to the input time of a
pulse from the inverter circuit 203.
As a circuit of the TDC, a delay line scheme in which a delay line
of buffer circuits connected in series is used to form a delay
circuit, a looped TDC scheme in which a circuit of delay lines
connected in a loop is used, or the like may be used, for example.
While other schemes may be used for the circuit of the TDC, a
scheme that can achieve time resolution that is greater than or
equal to the time resolution of the photoelectric conversion
element 101 is preferable in order to ensure sufficient time
resolution.
A digital signal acquired by the TDC is held in one or a plurality
of memories. When the number of memories is plural, it is possible
to cause any one of the plurality of memories to selectively output
a signal to the signal line 107 by supplying a plurality of control
signals SEL to the selector circuit 206.
FIG. 4 is a schematic diagram of the photo-detection device in the
present embodiment. The photo-detection device 1010 is formed with
a plurality of substrates being stacked. For example, the
photo-detection device 1010 includes a substrate (first
semiconductor substrate) 10 in which a plurality of photoelectric
conversion elements 101 are formed and a substrate (second
semiconductor substrate) 20 in which a plurality of pixel signal
processing units 102 are formed. The substrate 10 and the substrate
20 are attached at a junction interface. The junction interface is
formed of a metal such as copper and an insulating member such as
an oxide film. The metal forming the junction interface may form a
wiring connecting an element arranged on the substrate 10, such as
the photoelectric conversion element 101, to a circuit arranged on
the substrate 20, such as a counter.
Optical members such as color filters, micro-lenses, or the like
are arranged on a first primary face, that is, a light incidence
face of the substrate 10, and circuit elements forming the
photoelectric conversion element 101 are formed on a second primary
face of the substrate 10. The substrate 20 is stacked on the second
primary face of the substrate 10. A circuit group forming one pixel
100 is divided and formed in the substrate 10 and the substrate 20.
Thereby, it is possible to prevent an increase in the area of the
photo-detection device in a plan view while realizing an increase
in the speed or an increase in the size of a digital circuit
including the counter circuit. Note that the photoelectric
conversion element 101 and the pixel signal processing unit 102 may
be arranged side-by-side on a single substrate.
With reference to FIG. 5, FIG. 6, and FIG. 7, the configuration of
the photoelectric conversion element 101 in the present embodiment
will be described in detail. FIG. 5 is a plan view of the second
primary face of the photoelectric conversion element 101, and FIG.
6 is a plan view of the first primary face of the photoelectric
conversion element 101. Further, FIG. 7 is a sectional view taken
along a break line V-V of the photoelectric conversion element 101
in FIG. 5.
In the following description, an electron is used as a signal
charge. A semiconductor region of a first conductive type in which
first polarity carriers are the majority carriers is an N-type
semiconductor region, and a semiconductor region of a second
conductive type in which second polarity carriers are the majority
carriers is a P-type semiconductor region. Note that a hole may be
used as a signal charge, and in such a case, the N-type and the
P-type will be opposite.
The photoelectric conversion element 101 is formed in the substrate
10, and the substrate 10 includes a P-type semiconductor substrate
110, a light-shielding member 111, and an optical member 112. In
the semiconductor substrate 110, a rectangular pixel isolation
region 9 is formed in a plan view in the semiconductor substrate
110. In the region surrounded by the pixel isolation region 9, an
N-type semiconductor region (first semiconductor region) 1a, an
N-type semiconductor region (second semiconductor region) 3a, an
N-type semiconductor region (third semiconductor region) 5a, the
first transfer gate (first gate) 2, and the second transfer gate
(second gate) 4 are formed. The pixel isolation region 9 is used
for isolating the adjacent photoelectric conversion elements 101
from each other.
The N-type semiconductor region 1a (PD) forms the photoelectric
conversion unit 1 together with the P-type semiconductor substrate
110 and accumulates charges generated by photoelectric conversion.
Further, in a plan view, a P-type semiconductor region 1b is formed
in a region which is on the N-type semiconductor region 1a and does
not overlap the first transfer gate 2. The P-type semiconductor
region 1b reduces unnecessary charges that may occur at the surface
of the semiconductor substrate 110. In particular, by increasing
the impurity concentration of the P-type semiconductor region 1b,
it is possible to prevent depletion at the semiconductor interface,
and it is possible to avoid occurrence of unnecessary charges and
reduce the occurrence speed of a dark current.
The first transfer gate 2 is formed so as to overlap a part of each
of the N-type semiconductor regions 1a and 3a in a plan view. The
first transfer gate 2 controls the height of a potential barrier
near the semiconductor interface directly under the first transfer
gate 2. By controlling the voltage applied to the first transfer
gate 2, one or more signal charges are transferred from the N-type
semiconductor region 1a to the N-type semiconductor region 3a.
The N-type semiconductor region 3a (MEM) forms the charge
accumulation unit 3 together with the P-type semiconductor
substrate 110 and restricts the amount of charges transferred from
the N-type semiconductor region 1a to the N-type semiconductor
region 5a. It is preferable that the N-type semiconductor region 3a
be smaller than the N-type semiconductor region 1a in a plan view
and the number of charges that can be accumulated in the N-type
semiconductor region 3a (saturation charge amount) be less than the
number of charges that can be accumulated in the N-type
semiconductor region 1a (saturation charge amount). A P-type
semiconductor region 3b is formed in a region which is on the
N-type semiconductor region 3a and does not overlap the first
transfer gate 2 and the second transfer gate 4. In the same manner
as the P-type semiconductor region 1b described above, the P-type
semiconductor region 3b reduces unnecessary charges that may occur
on the surface of the semiconductor substrate 110.
The second transfer gate 4 is formed so as to overlap a part of
each of the N-type semiconductor regions 3a and 5a in a plan view.
The second transfer gate 4 controls the height of a potential
barrier near the semiconductor interface directly under the second
transfer gate 4. By controlling the voltage applied to the second
transfer gate 4, one or more signal charges are transferred from
the N-type semiconductor region 3a to the N-type semiconductor
region 5a.
P-type semiconductor regions 5b and 5c and N-type semiconductor
regions 5d and 5e are formed inside the N-type semiconductor region
5a. The P-type semiconductor region 5c forms the anode of an
avalanche diode, and the N-type semiconductor region 5d forms the
cathode of the avalanche diode. The N-type semiconductor region 5a
and the P-type semiconductor region 5b guide a signal charge to the
avalanche diode. An electric field between the P-type semiconductor
region 5c and the N-type semiconductor region 5d causes charge
multiplication due to avalanche multiplication. The N-type
semiconductor region 5e mitigates the electric field of the
avalanche diode. Although not illustrated in FIG. 7, a quench
circuit is further formed in the semiconductor substrate 110, and
the quench circuit may be electrically connected to the N-type
semiconductor region 5d of the avalanche diode.
The light-shielding member 111 is provided on the first primary
face of the photoelectric conversion element 101 and has an opening
111a at a position corresponding to the photoelectric conversion
unit 1 in a plan view. The optical member 112 is formed of a
transmissive material and may include a micro-lens 112a, a color
filter, or the like. The micro-lens 112a collects an incident light
from the backside of the substrate 10 and guides the collected
light to the photoelectric conversion unit 1 via the opening 111a.
The region other than the photoelectric conversion unit 1 is
shielded form light by the light-shielding member 111, and no light
enters the charge accumulation unit 3 and the charge multiplication
unit 5.
FIG. 8 is a timing chart illustrating the operation of the
photo-detection device in the present embodiment and illustrates
the electric potential of each of the first transfer gate 2 (TX1),
the second transfer gate 4 (TX2), the N-type semiconductor region
5d of the charge multiplication unit 5 (AD), the semiconductor
substrate 110 (SUB), the N-type semiconductor region 1a of the
photoelectric conversion unit 1 (PD), and the N-type semiconductor
region 3a of the charge accumulation unit 3 (MEM). The horizontal
axis represents time, and as one example, the cycle of readout and
detection of signal charges (time t0 to t5) may be several
nanoseconds.
On and before time t0, the N-type semiconductor region 1a (PD)
accumulates charges based on an incident light. The voltage control
unit 80 reduces the electric potential VH and thereby reduces the
electric potential of the N-type semiconductor region 5d (AD) to
stop the avalanche operation of the charge multiplication unit 5.
At this time, each electric potential of the control signals TX1
and TX2 and the semiconductor substrate 110 (SUB) may be 0 V, for
example. Further, the electric potentials of the N-type
semiconductor region 1a (PD) and the N-type semiconductor region 3a
(MEM) may be 1.5 V and 3 V, respectively.
At time t0, the voltage control unit 80 controls the electric
potential VH to set the electric potential of the N-type
semiconductor region 5d (AD) to 18 V, for example. Thereby, the
charge multiplication unit 5 is ready for performing avalanche
operation.
In the period of time t1 to t2, the control signal TX1 is
controlled to the high level (for example, 3V), and the first
transfer gate 2 is turned on. Thereby, some of signal charges
accumulated in the N-type semiconductor region 1a (PD) are
transferred to the N-type semiconductor region 3a (MEM). Further,
in response to an increase in the electric potential of the first
transfer gate 2, the electric potentials of the N-type
semiconductor region 1a (PD) and the N-type semiconductor region 3a
(MEM) also increase.
At time t2, the control signal TX1 is controlled from the high
level to the low level, and the first transfer gate 2 is turned
off. Several signal charges are accumulated in the N-type
semiconductor region 3a (MEM). Here, the number of signal charges
may not be correctly counted if a plurality of signal charges are
transferred to the charge multiplication unit 5 and
avalanche-multiplied at the same time. Thus, the number of signal
charges accumulated in the N-type semiconductor region 3a (MEM) is
preferably small, and more preferably 1 electron (1 e.sup.-) or 0
electron (0 e.sup.-), that is, less than two.
When one electron is transferred from the N-type semiconductor
region 1a (PD) to the N-type semiconductor region 3a (MEM), the
electric potential of the N-type semiconductor region 1a (PD)
increases by an electric potential corresponding to 1 electron (1
e.sup.-), and the electric potential of the N-type semiconductor
region 3a (MEM) decreases by an electric potential corresponding to
1 electron (1 e.sup.-).
In the period of time t3 to t4, the control signal TX2 is
controlled from the low level to the high level, and the second
transfer gate 4 is turned on. When a signal charge is transferred
from the N-type semiconductor region 3a (MEM) to the N-type
semiconductor region 5a, the signal charge is accelerated when
passing through an intense electric filed portion between the
P-type semiconductor region 5c and the N-type semiconductor region
5d and avalanche-multiplied. An avalanche current flows between the
N-type semiconductor region 5d and the P-type semiconductor region
5c, and the electric potential of the N-type semiconductor region
5d decreases. Then, due to the effect of the quench circuit 6, the
electric potential of the N-type semiconductor region 5d returns to
the electric potential obtained before the avalanche multiplication
has occurred. The counter circuit 204 of the pixel signal
processing unit 102 counts a change in the electric potential
occurring in the N-type semiconductor region 5d as a pulse.
As described above, the number of signal charges accumulated in the
N-type semiconductor region 3a (MEM) of the charge accumulation
unit 3 is less than the number of signal charges accumulated in the
N-type semiconductor region 1a (PD) of the photoelectric conversion
unit 1. It is desirable that the number of signal charges
accumulated in the N-type semiconductor region 3a (MEM) be one or
zero. Thereby, a plurality of signal charges are
avalanche-multiplied at the same time, and missing of counting of a
signal charge can be avoided. While the number of signal charges
accumulated in the N-type semiconductor region 3a (MEM) may be zero
when the second transfer gate 4 is turned on, the number of signal
charges can be counted as long as the signal charge is transferred
in one time of transfer out of multiple times of transfer. With the
transfer being repeated for multiple times, the time required for
counting signal charges may be longer. However, the transfer period
of signal charges (time t1 to t2, t3 to t4) is shorter than the
accumulation period of charges in the N-type semiconductor region
1a (PD) (on and before time t0). Further, by performing turning on
and off of the first transfer gate 2 and the second transfer gate 4
at a high speed, it is possible to avoid the overall processing
time being longer.
In the present embodiment, by separately performing accumulation of
signal charges in the photoelectric conversion unit 1 and counting
of signal charges in the charge multiplication unit 5, it is
possible to realize photo-detection at high sensitivity while
reducing noise due to unnecessary charges in the charge
multiplication unit 5. Further, the number of signal charges
accumulated in the charge accumulation unit 3 is less than the
number of signal charges accumulated in the photoelectric
conversion unit 1. Thus, the number of signal charges
avalanche-multiplied at the same time in the charge multiplication
unit 5 can be reduced, and the number of signal charges can be
counted more accurately. Furthermore, the number of signal charges
accumulated in the charge accumulation unit 3 is one or zero, that
is, less than two, and thereby the number of signal charges can be
counted much more accurately.
Further, signal charges accumulated in the photoelectric conversion
unit 1 are transferred to the charge multiplication unit 5 via the
charge accumulation unit 3. It is therefore possible to transfer
signal charges from the charge accumulation unit 3 to the charge
multiplication unit 5 while performing accumulation of signal
charges by using long time exposure in the photoelectric conversion
unit 1.
Second Embodiment
FIG. 9 is a timing chart illustrating the operation of a
photo-detection device in the present embodiment and illustrates
the electric potential of each of the first transfer gate 2 (TX1),
the second transfer gate 4 (TX2), the N-type semiconductor region
5d of the charge multiplication unit 5 (AD), the semiconductor
substrate 110 (SUB), the N-type semiconductor region 1a of the
photoelectric conversion unit 1 (PD), and the N-type semiconductor
region 3a of the charge accumulation unit 3 (MEM). The present
embodiment will be described mainly for features which are
different from those of the first embodiment.
In the present embodiment, the pulse width of each of the control
signals TX1 and TX2 may be around 1 nanosecond, for example. In
particular, by reducing the pulse width of the control signal TX2
of the second transfer gate 4, it is possible to reduce the period
of an avalanche operation in the charge multiplication unit 5, and
it is possible to reduce noise due to avalanche multiplication of
unnecessary charges. For example, it is assumed that the number of
signal charges accumulated in the photoelectric conversion unit 1
corresponds to 10,000 electrons, and the time required for one
cycle of the operation from signal charge transfer in the
photoelectric conversion unit 1 to avalanche multiplication in the
charge multiplication unit 5 is 5 nanoseconds. In this case, the
time required for readout of signal charges accumulated in the
photoelectric conversion unit 1 will be only 10,000.times.5
nanoseconds=50 .mu.seconds, which is sufficiently shorter than
several 10 milliseconds of accumulation time. Therefore, when the
pulse width of the first transfer gate 2 and the second transfer
gate 4 is reduced, the time required for readout is not so longer
even when the number of times of readout is increased.
When the control signals TX1 and TX2 are driven at a high speed,
the rising edge and the falling edge of the waveform are no longer
steep, the electric potentials of the control signals TX1 and TX2
change in accordance with a sinewave. Even when the control signals
TX1 and TX2 are sinewaves, it is possible to drive the first
transfer gate 2 and the second transfer gate 4. In the timing chart
illustrated in FIG. 9, the control signals TX1 and TX2 are
sinewaves, and the control signal TX2 is driven with a delay of
around 1/2 cycle from the control signal TX1. The control signal
TX1 changes between -1 V and 1.5 V, for example, in the initial
state, and the peak electric potential increases at every
repetition of charge transfer. The control signal TX2 repeatedly
changes between -1 V to 3 V, for example.
On and before time t0, the N-type semiconductor region 1a of the
photoelectric conversion unit 1 (PD) accumulates charges based on
an incident light. The voltage control unit 80 reduces the electric
potential VH and thereby reduces the electric potential of the
N-type semiconductor region 5d of the charge multiplication unit 5
to stop the avalanche multiplication in the charge multiplication
unit 5. At this time, the electric potential of the N-type
semiconductor region 1a (PD) may be 1.5 V.
At time t0, the voltage control unit 80 controls the electric
potential VH to set the electric potential of the N-type
semiconductor region 5d (AD) to 18 V, for example. Thereby, the
charge multiplication unit 5 is in a state where avalanche
multiplication can be performed.
In the period of time t1 to t2, once the level of the control
signal TX1 exceeds a threshold, the first transfer gate 2 is turned
on. Thereby, some of signal charges accumulated in the N-type
semiconductor region 1a (PD) are transferred to the N-type
semiconductor region 3a (MEM). In response to an increase in the
electric potential of the first transfer gate 2, the electric
potential of the N-type semiconductor region 1a (PD) also
increases. The electric potential of the N-type semiconductor
region 3a (MEM) also increases. The control signal TX2 changes in a
phase opposite to the control signal TX1. Thus, the control signals
TX1 and TX2 are cancelled with each other in the N-type
semiconductor region 3a (MEM), and the electric potential in the
N-type semiconductor region 3a (MEM) will be small.
When the level of the control signal TX1 becomes lower than the
threshold, the first transfer gate 2 is turned off. A signal charge
of 1 electron (1 e.sup.-) or 0 electron (0 e.sup.-) is accumulated
in the N-type semiconductor region 3a (MEM). When one electron is
transferred from the N-type semiconductor region 1a (PD) to the
N-type semiconductor region 3a (MEM), the electric potential of the
N-type semiconductor region 1a (PD) increases by an electric
potential corresponding to 1 electron (1 e.sup.-), and the electric
potential of the N-type semiconductor region 3a (MEM) decreases by
an electric potential corresponding to 1 electron (1 e.sup.-).
At time t3, in response to the level of the control signal TX2
exceeding a threshold, and the second transfer gate 4 is turned on.
When a signal charge is transferred from the N-type semiconductor
region 3a (MEM) to the N-type semiconductor region 5a, the electric
potential of the N-type semiconductor region 3a (MEM) increases by
an electric potential corresponding to 1 electron (1 e.sup.-). The
signal charge transferred to the N-type semiconductor region 5a is
accelerated when passing through an intense electric filed portion
between the P-type semiconductor region 5c and the N-type
semiconductor region 5d and avalanche-multiplied. An avalanche
current flows between the N-type semiconductor region 5d (AD) and
the P-type semiconductor region 5c, and the electric potential of
the N-type semiconductor region 5d decreases. Then, due to the
effect of the quench circuit 6, the electric potential of the
N-type semiconductor region 5d (AD) returns to the electric
potential obtained before the avalanche multiplication has
occurred. The counter circuit 204 of the pixel signal processing
unit 102 counts a change in the electric potential occurring in the
N-type semiconductor region 5d (AD) as a pulse.
Subsequently, in the same manner, the first transfer gate 2 and the
second transfer gate 4 are sequentially turned on. Signal charges
are sequentially transferred from the photoelectric conversion unit
1 to the charge multiplication unit 5 via the charge accumulation
unit 3, and avalanche multiplication is performed thereon in the
charge multiplication unit 5. The first transfer gate 2 is turned
on, and the peak value of the electric potential in the N-type
semiconductor region 1a (PD) gradually increases every time signal
charges are transferred from the N-type semiconductor region 1a
(PD).
When the number of signal charges accumulated in the N-type
semiconductor region 1a (PD) is large, the electric potential of
the N-type semiconductor region 1a (PD) decreases. Thus, even when
the electric potential of the first transfer gate 2 is not so high,
signal charges are easily transferred from the N-type semiconductor
region 1a (PD). Here, the electric potential of the first transfer
gate 2 is set to be low to suppress signal charge transfer, and
thereby current consumption can be reduced. On the other hand, when
the number of signal charges accumulated in the N-type
semiconductor region 1a (PD) is small, the electric potential of
the N-type semiconductor region 1a (PD) becomes high. In such a
case, to transfer signal charges from the N-type semiconductor
region 1a (PD) to the N-type semiconductor region 3a (MEM), it is
preferable to set the peak value of the electric potential of the
first transfer gate 2 to be high. In the present embodiment, by
gradually increasing the peak value of the electric potential of
the control signal TX1 applied to the first transfer gate 2 from a
low value to a high value, it is possible to perform stable charge
transfer while reducing current consumption. For example, in the
initial signal transfer after the photoelectric conversion unit 1
accumulates signal charges, the peak electric potential of the
control signal TX1 may be set to a sufficiently low value, for
example, 1.5 V. The peak electric potential of the control signal
TX1 then increases as time elapses and may be increased to 3 V, for
example. Note that the peak value of the electric potential of the
control signal TX1 may be increased in the same manner by
increasing the offset electric potential of the sinewave with time.
Further, the peak value of the electric potential may be changed by
changing the amplitude of the control signal TX1.
The number of signal charges accumulated in the N-type
semiconductor region 3a (MEM) of the charge accumulation unit 3 is
less than the number of signal charges in the N-type semiconductor
region 1a (PD) of the photoelectric conversion unit 1 and may
preferably be one or zero. Further, in the N-type semiconductor
region 3a (MEM), the electric potentials of the control signals TX1
and TX2 are cancelled with each other. Thus, the electric potential
of the N-type semiconductor region 3a (MEM) only increases by an
electric potential corresponding to 1 electron. It is therefore
possible to transfer the signal charge from the N-type
semiconductor region 3a (MEM) to the N-type semiconductor region 5a
without changing the peak electric potential of the control signal
TX2 in the second transfer gate 4.
As described above, according to the present embodiment, together
with the effect and advantage in the first embodiment, it is
possible to reduce the time required for counting signal charges by
reducing the cycle of transfer and avalanche multiplication of
signal charges to several nanoseconds. Thus, noise can be
reduced.
Third Embodiment
Next, a photo-detection device in a third embodiment will be
described. FIG. 10 is a sectional view of photoelectric conversion
elements in the present embodiment. In the photoelectric conversion
element in the present embodiment, no second transfer gate is
formed between the N-type semiconductor region 3a of the charge
accumulation unit 3 and the N-type semiconductor region 5a of the
charge multiplication unit 5. Signal charge transfer from the
charge accumulation unit 3 to the charge multiplication unit 5 is
performed by reducing the electric potential when the first
transfer gate 2 is in an off-state to be lower than the electric
potential in the first and second embodiments. Other features are
the same as those of the photoelectric conversion element
illustrated in FIG. 7.
FIG. 11 is a timing chart illustrating the operation of the
photo-detection device in the present embodiment and illustrates
the electric potential of each of the first transfer gate 2 (TX1),
the N-type semiconductor region 5d of the charge multiplication
unit 5 (AD), the semiconductor substrate 110 (SUB), the N-type
semiconductor region 1a of the photoelectric conversion unit 1
(PD), and the N-type semiconductor region 3a of the charge
accumulation unit 3 (MEM). In the same manner as the second
embodiment, the pulse width of the control signal TX1 applied to
the first transfer gate 2 may be 1 nanosecond, for example.
Further, the time required for one cycle of the operation from
signal charge transfer in the photoelectric conversion unit 1 to
avalanche multiplication in the charge multiplication unit 5 is 5
nanoseconds, for example. Features different from those in the
timing chart (FIG. 9) in the second embodiment will be mainly
described below.
On and before time t0, the N-type semiconductor region 1a (PD)
accumulates charges based on an incident light, and the charge
multiplication unit 5 suspends the avalanche operation. At time t0,
the charge multiplication unit 5 is ready for performing avalanche
operation. In the period of time t1 to t2, once the level of the
control signal TX1 exceeds a threshold, the first transfer gate 2
is turned on. Thereby, some of signal charges accumulated in the
N-type semiconductor region 1a (PD) are transferred to the N-type
semiconductor region 3a (MEM). Further, in response to an increase
in the electric potential of the first transfer gate 2, the
electric potentials of the N-type semiconductor region 1a (PD) and
the N-type semiconductor region 3a (MEM) also increase.
In the period of time t2 to t3, in response to a decrease in the
electric potential of the control signal TX1, the electric
potential of the N-type semiconductor region 3a (MEM) also
decreases. When the electric potential of the N-type semiconductor
region (MEM) becomes sufficiently lower than the electric potential
of the semiconductor substrate 110 (SUB), signal charges start
moving from the N-type semiconductor region 3a (MEM) to the N-type
semiconductor region 5a. In the present embodiment, the electric
potential of the low level of the control signal TX1 may be -2 V,
for example. The signal charges transferred to the N-type
semiconductor region 5a are avalanche-multiplied in the charge
multiplication unit 5. Then, when the electric potential of the
control signal TX1 increases, and the difference between the
electric potentials of the N-type semiconductor region 3a (MEM) and
the semiconductor substrate 110 (SUB) becomes below the threshold,
signal charge transfer from the N-type semiconductor region 3a
(MEM) to the N-type semiconductor region 5a is no longer
performed.
Subsequently, in the same manner, the control signal TX1 is driven
with a sinewave at a constant cycle, and signal charges from the
N-type semiconductor region 1a (PD) are transferred to the N-type
semiconductor region 5a via the N-type semiconductor region 3a
(MEM). As the number of signal charges of the N-type semiconductor
region 1a (PD) decreases, the electric potential of the N-type
semiconductor region 1a (PD) increases. The vertical select circuit
103 can stably transfer signal charges while reducing current
consumption by gradually increasing the peak value of the electric
potential of the control signal TX1.
According to the present embodiment, since it is not necessary to
provide the second transfer gate between the N-type semiconductor
region 3a (MEM) and the N-type semiconductor region 5a, it is
possible to reduce the size of a pixel. Note that the control
signal TX1 applied to the first transfer gate 2 is not limited to a
sinewave but may be a square wave in the same manner as in the
first embodiment.
Fourth Embodiment
FIG. 12 is a timing chart illustrating the operation of a
photo-detection device in the present embodiment. In the present
embodiment, the control signal TX2 changes from the low level to
the high level in a predetermined transfer period. The
photo-detection device in the present embodiment will be described
below mainly for features which are different from those of the
first embodiment.
The operation from time t0 to t3 is substantially the same as the
operation illustrated in FIG. 8 in the first embodiment. In the
present embodiment, to avoid missing of counting of a signal charge
in the charge multiplication unit 5, the time interval of signal
charge transfer to the N-type semiconductor region 5a is set to be
longer than time period of an electric potential change in the
charge multiplication unit 5. For example, in the period of time t3
to t4, the vertical select circuit 103 gradually increases the
electric potential of the control signal TX2 from the low level to
the high level. A transfer period for controlling the control
signal TX2 from the low level to the high level can be
appropriately set and, preferably, be longer than at least a period
for controlling the control signal TX2 from the high level to the
low level. Even when a plurality of signal charges are accumulated
in the N-type semiconductor region 3a (MEM), signal charges are
transferred to the N-type semiconductor region 5a one by one, and
each of which is avalanche-multiplied. It is therefore possible to
avoid missing of counting a signal charge due to simultaneous
avalanche multiplication of a plurality of signal charges and
correctly count the number of signal charges.
As described above, according to the present embodiment, the second
transfer gate 4 transitions from the low level to the high level in
a transfer period of a predetermined length. It is therefore
possible to avoid a plurality of signal charges being
simultaneously transferred to the charge multiplication unit 5 and
count the number of signal charges correctly.
Fifth Embodiment
Next, a photo-detection device in the present embodiment will be
described mainly for features which are different from those in the
third embodiment.
In FIG. 11, as described above, the control signal TX1 is
controlled to a lower potential than the semiconductor substrate
110 (SUB), and thereby a signal charge is transferred from the
N-type semiconductor region 3a (MEM) to the N-type semiconductor
region 5a. Here, when a plurality of signal charges are accumulated
in the N-type semiconductor region 3a (MEM), if the plurality of
signal charges are simultaneously transferred to the N-type
semiconductor region 5a and avalanche-multiplied, missing of
counting of a signal charge may occur. The photo-detection device
in the present embodiment transfers signal charges one by one from
the N-type semiconductor region 3a (MEM) to the N-type
semiconductor region 5a by setting a longer time period of
transition of the control signal TX1 from the high level to the low
level. With such a configuration, it is possible to avoid missing
of counting of a signal charge and count the number of signal
charges correctly.
Note that the time period of transition of the control signal TX1
from the high level to the low level can be suitably set within a
range that does not cause missing of counting of a signal charge.
For example, the time period of transition of the control signal
TX1 from the high level to the low level may be set to be longer
than the time period of transition of the control signal TX1 from
the low level to the high level.
Sixth Embodiment
FIG. 13 is a sectional view of a photoelectric conversion element
in the present embodiment. In the present embodiment, the light
receiving face is provided on the opposite side of the light
receiving face of the photoelectric conversion element illustrated
in FIG. 7. Features different from those in the first embodiment
will be mainly described below.
In FIG. 13, the photoelectric conversion element 101 is formed in
the substrate 10, and the substrate 10 includes the P-type
semiconductor substrate 110, the light-shielding member 111, and
the optical member 112. The photoelectric conversion unit 1, the
first transfer gate 2, the charge accumulation unit 3, the second
transfer gate 4, and the charge multiplication unit 5 are formed in
the semiconductor substrate 110. Furthermore, the light-shielding
member 111 is formed above the semiconductor substrate 110 via an
interlayer insulating film. The light-shielding member 111 has the
opening 111a at a position corresponding to the photoelectric
conversion unit 1 in a plan view. The optical member 112 is formed
on the light-shielding member 111. The optical member 112 is formed
of a transmissive material and may include a micro-lens, a color
filter, or the like. The micro-lens collects an incident light from
the backside of the substrate 10 and guides the collected light to
the photoelectric conversion unit 1 via the opening 111a. The
region other than the photoelectric conversion unit 1 is shielded
form light by the light-shielding member 111, and no light enters
the charge accumulation unit 3 and the charge multiplication unit
5.
The substrate 20 on which pixel signal processing unit 102 is
formed may be attached to the underside of the substrate 10, that
is, the face on the opposite side of the incidence face (see FIG.
4). The substrate 10 and the substrate 20 are attached at the
junction interface. The junction interface is formed of a metal
such as copper and an insulating member such as an oxide film.
According to the present embodiment, the light-shielding member 111
and the optical member 112 are formed similarly on the face on
which elements such as the photoelectric conversion unit 1 are
formed in the semiconductor substrate 110. It is therefore possible
to reduce the number of processes in semiconductor manufacturing
compared to the first embodiment and reduce the manufacturing cost.
Further, when the size of a pixel is large, the pixel signal
processing unit 102 such as a counter may be formed on the same
face of the semiconductor substrate 110 as the face on which the
photoelectric conversion element 101 is formed.
Seventh Embodiment
An imaging system according to a seventh embodiment of the present
disclosure will be described by using FIG. 14. FIG. 14 is a block
diagram illustrating a configuration example of the imaging system
according to the present embodiment.
In the present embodiment, with reference to FIG. 14, another
example of a photo-detection system using the photo-detection
device 1010 of the first to sixth embodiments will be described. A
component having the same function as that in FIG. 1 to FIG. 13 is
labeled with the same reference, and description thereof will be
omitted or simplified.
First, with reference to FIG. 14, a distance detection system that
is one example of the photo-detection system will be described.
Note that the pixel 100 of the present embodiment has a TDC 209 and
a memory 210 instead of the counter circuit 204 of FIG. 3.
FIG. 14 is a block diagram of a distance detection system. The
distance detection system has a light source control unit 1301, a
light emitting unit 1302, an optical member 1303, a photo-detection
device 1010, and a distance calculation unit 1309.
The light source control unit 1301 controls driving of the light
emitting unit 1302. The light emitting unit 1302 is a light
emitting device that emits a light with short pulses (sequence) in
the capturing direction in accordance with a signal from the light
source control unit 1301.
A light emitted from the light emitting unit 1302 is reflected at a
subject 1304. The reflected light is received by a photoelectric
conversion unit 201 of the photo-detection device 1010 through the
optical member 1303 such as a lens. The photoelectric conversion
unit 201 outputs a signal based on an incident light, and the
signal is input to the TDC 209 via the inverter circuit 203.
The TDC 209 acquires, from the light source control unit 1301, a
signal indicating a timing of light emission from the light
emitting unit 1302. The TDC 209 compares a signal acquired from the
light source control unit 1301 with a signal input from the
inverter circuit 203. Thereby, the TDC 209 outputs, as a digital
signal, the time period from the time when the light emitting unit
1302 emits a pulse light to the time when the reflected light
reflected by the subject 1304 is received. A digital signal output
from the TDC 209 is held in the memory 210. This process is
repeated for multiple times, and the memory 210 can hold a digital
signal for the multiple times.
The distance calculation unit 1309 calculates a distance from the
photo-detection device 1010 to the subject 1304 based on the
plurality of digital signals held in the memory 210. Such a
distance detection system is applicable to an on-vehicle distance
detection device, for example. Note that a process performed by the
distance calculation unit 1309 is a process of a digital signal and
thus may be more generally referred to as a signal processing
unit.
Eighth Embodiment
An imaging system and a moving body according to an eighth
embodiment of the present disclosure will be described by using
FIG. 15A and FIG. 15B. FIG. 15A and FIG. 15B are diagrams
illustrating a configuration of a photo-detection system 1000 and a
moving body according to the present embodiment.
FIG. 15A is a block diagram illustrating one example of the
photo-detection system 1000 related to an on-vehicle camera. The
photo-detection system 1000 has the photo-detection device 1010
according to the first embodiment. The photo-detection system 1000
has an image processing unit 1030 that performs image processing on
a plurality of digital signals acquired by the photo-detection
device 1010. Furthermore, the photo-detection system 1000 has a
parallax calculation unit 1040 that calculates a parallax (a phase
difference of parallax images) from a plurality of image data
acquired by the image processing unit 1030.
Further, the photo-detection system 1000 has a distance measurement
unit 1050 that calculates a distance to the object based on the
calculated parallax and a collision determination unit 1060 that
determines whether or not there is a collision possibility based on
the calculated distance. Here, the parallax calculation unit 1040
and the distance measurement unit 1050 are an example of a distance
information acquisition unit that acquires distance information on
the distance to the object. That is, the distance information is
information on a parallax, a defocus amount, a distance to an
object, or the like.
The collision determination unit 1060 may use any of the distance
information to determine the collision possibility. The distance
information acquisition unit may be implemented by dedicatedly
designed hardware, may be implemented by a software module, or may
be implemented by a combination thereof. Further, the distance
information acquisition unit may be implemented by a Field
Programmable Gate Array (FPGA), an Application Specific Integrated
Circuit (ASIC), or the like or, in addition, may be implemented by
a combination thereof.
The photo-detection system 1000 is connected to the vehicle
information acquisition device 1310 and can acquire vehicle
information such as a vehicle speed, a yaw rate, a steering angle,
or the like. Further, the photo-detection system 1000 is connected
to a control ECU 1410, which is a control device that outputs a
control signal for causing a vehicle to generate braking force
based on a determination result by the collision determination unit
1060.
Further, the photo-detection system 1000 is also connected to an
alert device 1420 that issues an alert to the driver based on a
determination result by the collision determination unit 1060. For
example, when the collision probability is high as the
determination result of the collision determination unit 1060, the
control ECU 1410 performs vehicle control to avoid a collision or
reduce damage by applying a brake, pushing back an accelerator,
suppressing engine power, or the like. The alert device 1420 alerts
a user by sounding an alert such as a sound, displaying alert
information on a display of a car navigation system or the like,
providing vibration to a seat belt or a steering wheel, or the
like.
In the present embodiment, an area around a vehicle, for example, a
front area or a rear area is captured by using the photo-detection
system 1000. FIG. 15B illustrates the photo-detection system 1000
when a front area of a vehicle (a capturing area 1510) is captured.
The vehicle information acquisition device 1310 transmits an
instruction to the photo-detection system 1000 or the
photo-detection device 1010 so as to perform a predetermined
operation. With such a configuration, it is possible to further
improve the ranging accuracy. The vehicle may further have a
control unit that controls the vehicle, which is a moving body,
based on the distance information.
Although the example of control for avoiding a collision to another
vehicle has been described above, the photo-detection system 1000
is applicable to automatic driving control for following another
vehicle, automatic driving control for not going out of a traffic
lane, or the like. Furthermore, the photo-detection system 1000 is
not limited to a vehicle and can be applied to a moving body
(moving apparatus) such as a ship, an airplane, or an industrial
robot, for example. In addition, the imaging system can be widely
applied to a device which utilizes object recognition, such as an
intelligent transportation system (ITS), without being limited to
moving body.
According to the present embodiment, by using the photo-detection
device 1010 having the improved detection performance, it is
possible to provide a photo-detection system and a moving body of
higher performance.
Modified Embodiments
The present disclosure is not limited to the embodiments described
above, and various modifications are possible. For example, an
example in which a part of the configuration of any of the
embodiments is added to another embodiment or an example in which a
part of the configuration of any of the embodiments is replaced
with a part of the configuration of another embodiment is also one
of the embodiments of the present disclosure.
Embodiments of the present disclosure can also be realized by a
computer of a system or apparatus that reads out and executes
computer executable instructions (e.g., one or more programs)
recorded on a storage medium (which may also be referred to more
fully as a `non-transitory computer-readable storage medium`) to
perform the functions of one or more of the above-described
embodiments and/or that includes one or more circuits (e.g.,
application specific integrated circuit (ASIC)) for performing the
functions of one or more of the above-described embodiments, and by
a method performed by the computer of the system or apparatus by,
for example, reading out and executing the computer executable
instructions from the storage medium to perform the functions of
one or more of the above-described embodiments and/or controlling
the one or more circuits to perform the functions of one or more of
the above-described embodiments. The computer may comprise one or
more processors (e.g., central processing unit (CPU), micro
processing unit (MPU)) and may include a network of separate
computers or separate processors to read out and execute the
computer executable instructions. The computer executable
instructions may be provided to the computer, for example, from a
network or the storage medium. The storage medium may include, for
example, one or more of a hard disk, a random-access memory (RAM),
a read only memory (ROM), a storage of distributed computing
systems, an optical disk (such as a compact disc (CD), digital
versatile disc (DVD), or Blu-ray Disc (BD).TM.), a flash memory
device, a memory card, and the like.
Note that all of the embodiments described above are mere embodied
examples in implementing the present disclosure, and the technical
scope of the present disclosure should not be construed in a
limiting sense by these embodiments. That is, the present
disclosure can be implemented in various forms without departing
from the technical concept or the primary feature thereof.
According to the present disclosure, a photo-detection device and
an imaging system that can reduce noise due to unnecessary charges
can be provided.
While the present disclosure has been described with reference to
exemplary embodiments, it is to be understood that the disclosure
is not limited to the disclosed exemplary embodiments. The scope of
the following claims is to be accorded the broadest interpretation
so as to encompass all such modifications and equivalent structures
and functions.
This application claims the benefit of Japanese Patent Application
No. 2018-244016, filed Dec. 27, 2018 which is hereby incorporated
by reference herein in its entirety.
* * * * *